Hot isostatic pressing (HIP) is a technology that finds more and more widespread use. Hot isostatic pressing is for instance used in achieving elimination of porosity in castings, such as for instance turbine blades, in order to substantially increase their service life and strength, in particular the fatigue strength. Another field of application is the manufacture of products, which are required to be fully dense and to have pore-free surfaces, by means of compressing powder.
In hot isostatic pressing, an article to be subjected to treatment by pressing is positioned in a load compartment of an insulated pressure vessel. A cycle, or treatment cycle, comprises the steps of: loading, treatment and unloading of articles, and the overall duration of the cycle is herein referred to as the cycle time. The treatment may, in turn, be divided into several portions, or phases, such as a pressing phase, a heating phase, and a cooling phase.
After loading, the vessel is sealed off and a pressure medium is introduced into the pressure vessel and the load compartment thereof. The pressure and temperature of the pressure medium is then increased, such that the article is subjected to an increased pressure and an increased temperature during a selected period of time. The temperature increase of the pressure medium, and thereby of the articles, is provided by means of a heating element or furnace arranged in a furnace chamber of the pressure vessel. The pressures, temperatures and treatment times are of course dependent on many factors, such as the material properties of the treated article, the field of application, and required quality of the treated article. The pressures and temperatures in hot isostatic pressing may typically range from 200 to 5000 bars, and preferably from 800 to 2000 bars and from 300° C. to 3000° C., and preferably from 800° C. to 2000° C., respectively.
Today, there is also an increasing demand from HIP arrangement customers to be able to tailor or customize the treatment cycle with a high degree of temperature accuracy and stability and with possibilities of a very rapid and uniform cooling. For example, it may be desired to first increase the pressure and temperature to first pressure level and a first temperature level and to maintain the temperature and pressure at these levels during a first period of time. Thereafter, it may be desired to lower the temperature rapidly without causing any large temperature variations within the load compartment (i.e. that the temperature is reduced uniformly) in a controlled manner and to hold the temperature at a second temperature level during a second period of time with a high degree of temperature stability. It is also as mentioned important that the treated work piece or pieces are cooled in a uniform or homogenous manner to avoid any defects in the material since, in many kinds of metallurgical treatment, e.g. temperature variation within the work piece during the cooling will affect the metallurgical properties in a negative manner.
When the pressing of the articles is finished, the articles often need to be cooled before being removed, or unloaded, from the pressure vessel. As mentioned above, the cooling and the cooling rate may affect the metallurgical properties. For example, thermal stress (or temperature stress) and grain growth should be minimized in order to obtain a high quality material. Thus, it is desired to cool the material homogeneously and, if possible, to control the cooling rate. Many presses known in the art suffer from slow cooling of the articles, efforts have therefore been made to reduce the cooling time of the articles.
U.S. Pat. No. 5,123,832 discloses a hot isostatic press for achieving a more even cooling of the load, wherein a gas mixture is achieved by mixing, in an ejector, cold gas with hot gas from the furnace chamber. The temperature of the gas mixture which is ejected into the loading space is about 10% lower than the present temperature in loading space. The mixing of the cold gas and the hot gas in the ejector requires a considerable throttling or restriction for providing a good mixing effect. The inlet for the mixed gas into the loading space is thus very small, typically 100 mm in diameter, whereas the diameter of the loading space is typically about 1.2 m. Even though a satisfactory cooling may be achieved, this construction also has drawbacks. During the pressing operation, when the furnace chamber is to be heated, the heating of the furnace chamber, and the loading space in particular, would become extremely uneven because of the small inlet area to the loading space, unless heating elements are provided on the side of the furnace chamber. In many cases it is desirable to only have heating elements at the bottom portion of the furnace chamber, for, inter alia, reasons such as simplicity and cost-saving. Thus, there remains a need for a simple alternative which provides good mixing and which does not have the above constructional limitations.
In other prior art hot isostatic presses, a fan is mounted in the furnace chamber for circulating the pressure medium within the furnace chamber and enhance an inner convection loop, in which pressure medium has an upward flow through the load compartment and a downward flow along a peripheral portion of the furnace chamber. Typically, the fan is mounted at the bottom of the load compartment, in connection to the entrance opening for the pressure medium into the load compartment. That is, the fan is mounted below the load (in a vertical direction) at the pressure medium entrance into the load compartment to achieve that the flow of pressure medium passes the load. Thereby, it is possible to affect the cooling by operating the fan at different operation speeds.
However, in order to obtain a very rapid cooling in combination with the ability to hold the pressure medium at a given temperature with a high degree temperature stability within the load compartment (i.e. the whole load), a very large fan is required and, in turn, a powerful motor. This will of course require more space within the pressing arrangement, which entails that the load compartment instead will be smaller. Further, this solution will also require a heat exchanger to provide additional cooling of the pressure medium.
In U.S. Pat. No. 5,118,289, a hot isostatic press adapted to rapidly cool the articles after completed pressing and heating treatment by utilizing a heat exchanger is disclosed. The heat exchanger is located above the hot zone, in order be able to decrease the time for cooling of articles. Thereby, the pressure medium will be cooled by the heat exchanger before it makes contact with the pressure vessel wall. Consequently, the heat exchanger allows for an increased cooling capacity without the risk of overheating the wall of the pressure vessel. Further, as in conventional hot isostatic presses, the pressure medium is cooled when passing through a gap between the pressure vessel wall and the thermal barriers during cooling of articles. When the cooled pressure medium reaches the bottom of the pressure vessel, it re-enters the hot zone (in which the articles to be cooled are located) via a passage through the thermal barrier. If the heat exchanger is combined with a large fan to obtain the rapid cooling rate and capability to maintain a given temperature with a high degree of accuracy, the pressure medium can be circulated further through the lead compartment by operation of the fan mounted at the bottom of the load compartment close to the entrance for pressure medium.
However, this solution is associated with drawbacks. For example, the heat exchanger becomes hot during cooling of the pressure medium and the articles, and, in order to function as a booster during the cooling of articles, the heat exchanger must be cooled before the press may be operated to treat a new set of articles. Thus, the time between subsequent cycles is dependent on the cooling time of the heat exchanger.
Yet another approach could be to combine the fan with an ejector (and potentially also on heat exchanger). The ejector can be mounted to eject cold gas (i.e. pressure medium) in the intake of the fan and thereby a mix of warm and cold pressure medium can be created. The amount of cold pressure medium transported into the load compartment can be controlled by controlling the feeding of the ejector. One problem with this approach is that cold pressure medium always will be drawn into the inner convection loop as soon as circulation is started (by starting the fan). This will inevitably lead to high losses of power and may also affect the capacity of the heat exchanger in a negative way. Further, also with an ejector mounted such that cold pressure medium is provided to the intake of the fan, the fan will have to be large since very large amounts of pressure medium has to be transported into the lead compartment to obtain the desired rapid cooling and capability to maintain the temperature at a given level.
Consequently, despite all efforts that have been made within the art, there is still a need for an improved solution that can provide the desired rapid uniform, or homogenous, cooling and capability of holding or maintaining the temperature at a given temperature level without the above drawbacks.